This invention relates to the field of displays for electronic spectrum analyzers, and more particularly to a system of markers for use in reading out amplitude, frequency and time values and making comparisons between the values at two locations in a quasi-3-dimensional display of multiple frequency spectra of a time varying electronic signal under analysis.
Spectrum analyzers, also known as signal analyzers, are electronic instruments that provide frequency domain information about time varying electronic signals under analysis. Such instruments monitor an electronic signal to obtain time domain information, and then transform the time domain information into frequency domain information for presentation to a user. The user sees a series of amplitude versus frequency spectra that describe how the energy of the signal is distributed in terms of the different frequency components that make up that signal.
Time domain information consists of a series of amplitude measurements made at regular intervals. In the digital spectrum analyzer in which the present invention was first implemented, an analog input signal is sampled at regular intervals and the voltage present at each of these times is converted into a digital word. A discrete Fourier transformation (DFT) is then performed on a segment of this digitized time domain data in order to calculate the corresponding frequency domain information. Usually, some variation of the fast Fourier transform (FFT) is used to accomplish the DFT calculation.
Once this frequency domain information has been produced by the spectrum analyzer, it is usually displayed as an amplitude versus frequency graphical representation on some sort of display device, such as a raster scan display screen. It is well known to use a system of markers to perform readouts and delta-parameter measurements on this type of display.
If the spectrum analyzer is continuously monitoring a certain band of frequencies, it is desirable to be able to see a history of the spectra generated by the analyzer over time. This allows an operator to visually compare these numerous spectra with each other to identify changes in the signal that are occurring over longer periods of time than the sampled interval used to calculate the individual spectra. This can be accomplished by storing each spectrum after it is calculated and first displayed, and then redisplaying a number of these spectra in some way that allows an operator to monitor them simultaneously and make comparisons.
The digital spectrum analyzer in which the present invention first found embodiment is capable of producing two types of quasi-3-dimensional displays: waterfall displays and color spectrogram displays. A waterfall display consists of a large number of amplitude versus frequency spectral displays closely stacked together, providing for ready comparison between their features. In this display time and amplitude share an axis, creating an illusion of a third dimension through the appearance of a relief drawing.
The color spectrogram display shows a number of spectra which were generated over time as a series of colored lines. Color is used as a substitute for a third dimension, permitting numerous frequency spectra to be compressed into a small area and readily compared by the user. Each single line of the spectrogram display is a complete spectrum, with different frequencies being represented by different points along the line and the color of each point representing the amplitude of the signal at that frequency. The other axis represents time, with the individual complete spectra moving along this axis as successive spectra are calculated by the spectrum analyzer. Thus, in a dynamic mode of operation, as a new spectrum appears at one end of this display, all of the previously displayed spectra are moved up one line along the time axis, with the spectrum which represents the oldest data disappearing from the other end of the display (after the display is filled). In a static mode of operation, this flow of spectra is stopped for detailed analysis, or a series of spectra previously generated are recalled from memory and displayed for further analysis.
In the digital spectrum analyzer that embodies the present invention, the spectrogram display is adjacent to and aligned with an amplitude versus frequency display, so that the two displays share the same frequency axis. In this dual display, the amplitude versus frequency portion of the display may be seen as a detailed view of the current edge of the color spectrogram portion of the display. Alternatively, the spectrogram portion of the display may be viewed as a compressed history of the contents of the amplitude versus frequency portion of the display.
The color of each point on the spectrogram display indicates the amplitude or power of the signal at a particular frequency and time. The values on the time axis represent the times at which the time domain data was collected relative to a common reference time. The values along the frequency axis represent the different frequency bins of the FFT output. Because it is difficult for an operator to accurately locate the position along each axis that corresponds to a particular point of interest out in the middle of the spectrogram display, and because reading out values on these axes requires operator interpolation and the precision of any resulting reading is necessarily limited, what is desired is a way to read out precise values of amplitude, time, and frequency for particular points on the quasi-3-dimensional display to a desired level of precision and with a minimum of operator effort. What is also desired is a way to conveniently make comparisons in the values of amplitude, time, or frequency between any two points on the quasi-3-dimensional display.